In many industries today, such as the biomedical and aerospace industries, components experience severe service conditions and, thus, are often made from titanium alloys or superalloys. For example, turbine-powered aircraft contain critical rotating components in the engine, such as the fan, compressor and the turbine sections, that are made from titanium alloys and/or superalloys. Generally, such alloys are manufactured by secondary remelting processes, such as plasma arc cold hearth melting (PAM), electron beam cold hearth melting (EBM), vacuum arc remelting (VAR), and electroslag remelting (ESR). During the manufacturing of the components, quality control takes a significant role because failure of such components can lead to catastrophic loss of the complex system as well as other losses.
In quality control, one of the most important quality issues for titanium alloys and superalloys is melt-related inclusions. In this regard, inclusions can consist of unusually coarse segregated phases formed in the melt, or as exogenous materials having origins outside the deliberate alloy constituents. In the case of exogenous materials for investment castings, one type of inclusion is mold shell fragments. Mold shell fragments are inadvertently released from the ceramic shell mold during casting as a result of high thermal stresses and erosion of the mold by the molten metal. The ceramic mold innermost layer (that faces the molten metal) typically contains rare earth metal oxide(s), such as erbia, that are utilized because of their high melting point and chemical compatibility with the reactive titanium melt. Upon release, the mold shell fragments may be incorporated into the body of the casting itself, and thereby become inclusion defects.
Another type of exogenous inclusion, peculiar to titanium and other reactive metal alloys with solvus temperatures that rise with interstitial oxygen, nitrogen or carbon content, is “hard alpha”, also known as Type I inclusions. Hard alpha inclusions originate within such alloys during process operations, such as welding, flame cutting, grinding, cutting and even furnace air leaks, that expose the molten alloy to elements in air, particularly oxygen, nitrogen, and carbon. When such exposure occurs, the alloy takes the elements into solution where the elements simultaneously stabilize and embrittle the alpha phase of the alloy. By stabilizing and embrittling the alpha phase, a defect is created within the alloy that is very similar in most other respects to the base alloy. Particulate debris from such operations can inadvertently migrate to the casting furnace and enter the ceramic shell mold as the casting pour takes place. Because hard alpha inclusions have a melting point exceeding that of the clean alloy, hard alpha inclusions can survive exposure to the melt, enter the mold and become a brittle inclusion. Additionally, hard alpha inclusions can enter the primary metal supply stream and unknowingly become part of the melt stock.
As stated, hard alpha inclusions originate during certain process operations within titanium and other reactive metal alloys with solvus temperatures displaying a positive slope as oxygen, nitrogen or carbon are added. Such process operations are integral to other process streams at the foundry where the components are manufactured and, as such, the process operations cannot be totally eliminated or isolated from the casting activity. As a result, detailed contamination control plans are typically implemented to prevent the generation and introduction of hard alpha debris into the foundry. Such contamination control plans, however, have a number of drawbacks. While contamination control plans aid in preventing the generation and introduction of hard alpha debris, such control plans generally do not remove hard alpha debris that actually do form or become introduced from operations external to the foundry. Also, contamination control plans typically add cost to the manufacture of the components, and add time required in the production schedule of the components. Additionally, such contamination control plans are generally difficult to enforce among manufacturers, and cannot be easily validated. In this regard, because of the limitations of contamination control plans and the associated risk of component failure, the design of components made from such alloys still typically accounts for the presence of hard alpha inclusions.